The invention relates to radio transmitters for transmitting radio signals which vary in amplitude as well as in phase or frequency.
From the article: Asad A. Abidi: xe2x80x9cLow-Power Radio-Frequency ICs for Portable Communicationsxe2x80x9d, Proceedings of the IEEE, Vol. 83 No. 4, April 1995, pp. 544-569, radio transmitters are known, for example in mobile telephones, where signals representing a desired transmitted radio signal are processed in a Digital Signal Processor and delivered from the processor to a transmitter radio frequency (RF) output unit as digital signals in quadrature form; compare for example FIG. 13.
It is a disadvantage in the transmitters thus known that the non-linearities of the power amplifier distort the transmitted signal.
From the article: V. Petrovic: xe2x80x9cReduction of Spurious Emission from Radio Transmitters by Means of Modulation Feedbackxe2x80x9d, IEE Conference on Radio Spectrum Conservation Technique, September 1983, pp. 44-49, a Single-Sideband (SSB) Radio Transmitter is known, in which two output quadrature signals are generated from the output RF signal, and two input quadrature signals are generated from an input signal as well. The quadrature output signals are demodulated by two quadrature carrier signals generated digitally, and are fed back and compared with the input quadrature signals. Difference quadrature signals are amplified and modulated with the carrier signals and summed to form the output RF signal.
It is a disadvantage in this known transmitter that the input quadrature signals are generated from a complete, modulated input signal by means of a phase-shift circuit. The article mentions the difficulties experienced when designing a reliable wide-band 90xc2x0 phase-shift network.
From the article: Hiroaki Kosugi et al.: xe2x80x9cA High-Efficiency Linear Power Amplifier Using an Envelope Feed-Back Methodxe2x80x9d, Electronics and Communication in Japan, Part 2, Vol. 77, No. 3, 1994, pp. 50-57, a Power Amplifier is known, in which input signals are fed to a power amplifier in quadrature form, that is generated by a quadrature mixer not specified in detail. An envelope signal is derived mathematically from the input quadrature signals, and is used as independent variable in an envelope feed-back loop controlling the envelope of the power output signal. No phase feed-back is disclosed.
It is a disadvantage in this known amplifier that the phase distortion of the power amplifier is not controlled or overcome.
From the article: Leonard R. Kahn: xe2x80x9cSingle-Sideband Transmission by Envelope Elimination and Restorationxe2x80x9d, Proceedings of the I.R.E., 1952, pp. 803-806, an SSB Transmitter is known, wherein the phase and the amplitude components of an input signal are separated and amplified independently before being combined in a final output stage.
It is a serious disadvantage in this transmitter that modulating the amplitude at a high power level tends to generate considerable amounts of spurious signals.
From the article: V. Petrovic and W. Gosling: xe2x80x9cPolar-Loop Transmitterxe2x80x9d, Electronics Letters, 10 May 1979 Vol. 15 No. 10, pp. 286-288, a Polar Modulation Feedback Linearisation System for an SSB Transmitter is known, wherein the modulated output from a Radio Frequency Power Amplifier (RF PA) is being controlled by means of two closed loops, each having the same modulated Intermediate Frequency (IF) Signal as the controlling variable. A first loop controls the Amplitude Modulation (AM) or the xe2x80x9cenvelopexe2x80x9d of the RF output signal relative to the IF signal, and a second loop controls the Phase Modulation (PM) of the RF output signal relative to the IF signal. Polar representation of the RF signal vector is used in the article, the vector angle representing the signal phase and the vector magnitude representing the signal amplitude.
Thus, this polar modulation feedback linearization system uses a fully modulated IF signal which is demodulated into an amplitude signal and a phase signal, respectively. It is a disadvantage that the analog control signals for the amplitude and phase loops thus have to be extracted from the IF signal.
The invention provides a digital signal set containing amplitude as well as phase or frequency modulation information, the signal set enabling transfer of this information from a digital domain to an analog domain in an efficient way, the signals being comparatively easily generated in the digital domain and at the same time being suited for conversion into an analog signal set in a way reducing efficiently the risk of xe2x80x9ccross-talkxe2x80x9d between the amplitude modulation and the phase or frequency modulation, and enabling a very efficient linearisation of a simple and energy-efficient analog output stage in a radio transmitter.
The invention provides a set of digital signals; a radio transmitter; and a method for transforming modulation information from a digital signal processing system into analog signals for controlling analog modulator and amplifier circuitry.
In a digital signal set containing amplitude as well as phase or frequency modulation information for compound modulating a carrier wave, the set comprises the following three digital signals:
a first and a second digital signal representing a desired phase angle of the carrier wave; and
a third digital signal representing a desired amplitude of the carrier wave.
Hereby, modulation information is provided which is suitable for driving particularly simple modulator circuits while obtaining high quality modulation with a minimum of cross-talk between the different types of modulation, while the information is still simple to establish in a digital signal processing system.
Cross-talk between the amplitude and phase or frequency modulation, respectively, is primarily being avoided due to the fact that the amplitude modulation information and the phase or frequency modulation information, respectively, are separated from one another in the digital domain, such that the digital-to-analog conversion may be made on the separated modulation information signals rather than on a compound signal containing both types of modulation information. Retrieval of the two types of modulation information from one and the same compound analog signal of the prior art will not be possible with state-of-the-art analog circuitry without the use of very elaborate and expensive circuits.
It is preferred that the first and second digital signals correspond to each their component of a desired phase unit vector in a Cartesian coordinate system.
Hereby, phase or frequency information is provided which is particularly suited for being transformed into analog phase or frequency information in a simple modulator circuit.
The invention provides a method for amplitude and phase or frequency modulating an output signal from an amplifier circuit such as a radio transmitter output stage, the method permitting efficient and component-saving use of contemporary digital signal processing systems together with analog aerial output circuitry and at the same time eliminating the disadvantages of the prior art.
A method for transforming modulation information from a digital signal processing system into analog signals suitable for controlling analog modulator and amplifier circuitry, the information comprising amplitude modulation information as well as phase or frequency modulation information for compound modulating a carrier wave, comprises the steps of:
providing a first and a second digital signal representing a desired phase angle of the carrier wave;
converting each of the first and second digital signals into corresponding first and second analog signals;
mixing the first and second analog signals with first and second oscillator signals to obtain an analog, phase or frequency modulated signal;
providing a third digital signal corresponding to the desired amplitude of the carrier wave;
converting the third digital signal into a corresponding analog amplitude signal; and
feeding the analog phase or frequency modulated signal and the analog amplitude signal to said analog modulator and amplifier circuitry.
Hereby, analog signals containing the desired analog as well as phase or frequency modulation information have been provided by means of very simple circuitry, the signals having very low levels of amplitude distortion and phase or frequency distortion, respectively.
The three digital signals mentioned may easily be provided by a digital signal processing system of the state of the art when adequately programmed, even if constraints are put to the first and the second digital signal as to representing a phase or frequency vector of unity magnitude, or at least of a constant magnitude.
It is generally preferred that each of the first and second digital signals, or each of the first and second analog signals, respectively, correspond to a respective component of a desired phase unit vector in a Cartesian coordinate system.
In a cartesian coordinate system, an optimal discrimination or resolution of the phase information is obtained, relative to the discrimination or resolution of the components of the phase vector.
It is likewise preferred that the first and second oscillator signals are in quadrature.
By this measure, the local oscillator signals may be generated more easily with adequate precision, for example by frequency dividing a master oscillator square wave signal into two quadrature square wave signals, or by generating by means of a CR coupling and an RC coupling (resistance-capacitance couplings) a +45xc2x0 and a xe2x88x9245xc2x0 shifted sine wave from a master sine wave.
Preferably, the analog, phase or frequency modulated signal has a constant amplitude.
Hereby, any risk of amplitude noise from the phase or frequency modulated signal affecting or contaminating the amplitude modulation of the output signal is significantly reduced. In addition, a phase detector of a phase locked loop will perform better on a modulated signal having a constant amplitude.
The invention provides a radio transmitter corresponding to the digital signal set and the method of the invention.
A radio transmitter comprising a digital signal processing system as well as analog signal processing circuitry and an analog output stage, comprises:
first and second digital-to-analog conversion circuits for converting first and second digital quadrature signals from the digital signal processing system into corresponding first and second analog quadrature signals;
a first mixer circuit with an associated first local oscillator circuit for mixing the first and second analog quadrature signals with first and second quadrature signals from the local oscillator into a constant amplitude, phase or frequency modulated driver signal;
frequency converter circuitry for converting the driver signal to a desired output frequency band;
a power amplifier for amplifying the converted signal into a transmitter output signal;
a third digital-to-analog conversion circuit for converting a digital amplitude signal from the digital signal processing system into a corresponding analog amplitude signal; and
an amplitude modulating circuit for modulating the amplitude of the transmitter output signal in dependency of the analog amplitude signal.
With the invention, simple modulator circuitry is obtained, in which the analog output stage may be supplied with digital modulation signals which are easily generated within the digital signal processor, while enabling modulation of the transmitter output signal with very low phase or frequency distortion.
It is generally preferred that the frequency converter circuitry together with the power amplifier forms part of a phase locked loop.
The advantages known per se of the phase locked loop enables full benefit to be had of the advantageous phase modulation circuitry of the invention; in particular, it enables a very high phase linearity of the output power amplifier, which tends to minimize any phase distortion arising from imperfections in the power amplifier, or from amplitude modulating the power amplifier.
It is further preferred that the frequency converter circuitry comprises in a main signal path a voltage controlled oscillator controlling a power amplifier, and in a feed-back path a divider circuit, or a second mixer circuit with an associated second local oscillator circuit.
This embodiment provides for a further simple and stable phase locked loop.
In a preferred embodiment, the amplitude modulating circuit comprises a closed amplitude control loop.
With the invention, the amplitude linearity of the analog circuitry is substantially improved.
In a particularly preferred embodiment, the power amplifier forms part of the amplitude control loop.
In this embodiment, the resulting amplitude linearity of the output stage I generally depends on characteristics of the amplitude control loop rather than characteristics of the power amplifier. This enables use of power amplifiers with less ideal amplitude linearity, and among such power amplifiers are particularly power efficient amplifiers. The use of such power efficient amplifiers are generally advantageous, and particularly so in battery powered equipment such as mobile equipment.
xe2x80x9cCross-talkxe2x80x9d from the amplitude modulation to the phase or frequency modulation (or: phase or frequency distortion, respectively, generated by the amplitude modulation) in amplifiers processing compound modulated signals always is present to some small extent, due to the very fact that the shift of the amplifier gain introduced by the shift of the modulated amplitude in itself affects the momentary phase or frequency, respectively, of the signal.
Substantial cross-talk from the amplitude modulation to the phase or frequency which normally is encountered when using a power amplifier with a poor amplitude linearity for amplifying signals that are both amplitude and phase or frequency modulated, is however avoided in the embodiments of the invention with both an amplitude feedback loop and a phase locked loop, because the phase distortion generated from the amplitude modulated signal by the amplitude linearity defects of the power amplifier are cancelled by the phase linearity feedback of the phase locked loop.
This is so because the phase distortion is generated inside of the phase locked loop, and thus is a part of the circuit which is controlled by the phase locked loop.
Further, the invention provides an advantageous use of the digital signal set in a mobile radio transmitter, preferably a mobile radio transmitter in a cellular telephone.
This use makes possible a lower component count in the modulator circuitry of a mobile radio transmitter, thus providing for lower weight, lower price and lower power consumption.
Finally, the invention provides an advantageous use of the radio transmitter of the invention in mobile equipment, preferably a mobile, cellular telephone.
This use makes possible a lower component count and a higher power efficiency in the output stage with similar or better modulation quality, thus providing for lower weight, lower price, lower power consumption and higher signal quality.